A common method of providing strong threads in plastic sections is achieved through the use of internally threaded metallic inserts. Such products are commonly available from many domestic as well as foreign sources, such as Yardley Inserts, P.S.M. International, Groove-Pin Corporation, and others. These inserts are usually manufactured from a non-ferrous material such as brass, but may also be manufactured from steel, stainless steel, or aluminum, and are tubular devices containing internal threads. They are either molded into plastic sections or installed in plastic sections having a suitable receiving hole or boss. The installation process consists of pressing the inserts into the receiving hole by means of an axial force sufficient to develop the required installation pressure, and may include heat, ultrasonic insertion, or a combination of these procedures.
The outside diameters of the inserts typically have ribs or grooves that provide a high resistance to pull-out due to the flow of plastic material around and into the ribs or grooves during the installation process. The ribs or grooves are typically used in combination with knurls that develop a high torque resistance between the insert and the plastic section. When the insertion process employs only the application of axial force without heat, the resilience of the material comprising the inside wall of the receiving hole can provide sufficient force to grip the ribs and knurls after the insert has been driven into the hole.
Heretofore, such inserts have been manufactured from bar stock on screw machines at production rates ranging from 800 to 1500 pieces per hour. The consumption of raw material is significant in producing these inserts, as the finished parts typically represent only 50% of the weight of the unmachined blank. Having machined (tapped) internal threads, there also exists the possibility of such inserts being shipped with substandard or missing threads due to tap wear and/or breakage, which can cause great hardship to the end user during final assembly when such defective or missing threads are discovered. Also, the internal threads of such inserts have been found to entrap miscellaneous materials, such as machining chips, which again can be a considerable drawback to end product quality.
Furthermore, inserts which are installed without the use heat or ultrasonics are limited to use in plastic materials which have sufficient resilience to grip the ribs and knurls after the deformation of insertion. This may preclude using these inserts in harder or filled plastics which usually have high melting points and are designed for severe applications. There is therefore a need in the art for a more cost-effective and reliable metallic insert for plastic sections which may be used in a wide variety of plastic materials.
The most pertinent patent prior art of which the applicant is aware includes U.S. Pat. No. 3,433,119 issued to Ballantyne et al on Mar. 18, 1969. This reference discloses a longitudinal sheet metal insert with internal and external punched-out barbs for insertion into a plastic material prior to receiving a threaded member. U.S. Pat. No. 3,496,800 discloses a triangularly-shaped sleeve which is internally threaded by pressing a tubular member against an internally-positioned threaded die. U.S. Pat. No. 3,314,509 issued to Olson on Mar. 23, 1943 discloses the use of a longitudinally-folded sheet metal fastener having interrupted threads which are embossed into the sheet metal. U.S. Pat. No. 3,006,231 issued to Kahn on Oct. 31, 1961 discloses an internally-threaded nut formed from longitudinally-folded sheet metal where the threads are preformed on the sheet. U.S. Pat. No. 3,532,323 issued to Uhen on Oct. 6, 1970 discloses a laterally-folded sheet metal nut having slots cut into the sheet which receive and engage the external threads of a bolt. U.S. Pat. No. 5,419,043 issued to Laue on May 30, 1995 discloses a longitudinally-folded fastener with thread indentations that utilizes an alignment tab. U.S. Pat. No. 1,111,749 issued to C. Joseph on Sep. 29, 1914 discloses a sheet metal insert, longitudinally folded, which employs outward-facing barbs and thread-engaging cutouts. U.S. Pat. No. 4,430,033 issued to McKewan on Feb. 7, 1984 discloses a sheet metal insert for foam plastic which includes an outward-facing rippled surface and interfacing thread-engaging ribs.
However, none of the above teachings suggest the use of forming an internally-threaded insert by progressive impact stamping of sheet material and then folding the stamped sheet. The use of this method of fastener formation permits economies of manufacture which are heretofore unattainable by any of the known prior art processes for forming threaded inserts.